Change (void) casts to TEST_IGNORE_NODISCARD, as requested by Eric. Reviewed as https://reviews.llvm.org/D40065

llvm-svn: 318804
diff --git a/libcxx/test/std/strings/basic.string/string.access/at.pass.cpp b/libcxx/test/std/strings/basic.string/string.access/at.pass.cpp
index 8bfb829..16121cb 100644
--- a/libcxx/test/std/strings/basic.string/string.access/at.pass.cpp
+++ b/libcxx/test/std/strings/basic.string/string.access/at.pass.cpp
@@ -35,7 +35,7 @@
     {
         try
         {
-            (void)s.at(pos);
+            TEST_IGNORE_NODISCARD s.at(pos);
             assert(false);
         }
         catch (std::out_of_range&)
@@ -44,7 +44,7 @@
         }
         try
         {
-            (void)cs.at(pos);
+            TEST_IGNORE_NODISCARD cs.at(pos);
             assert(false);
         }
         catch (std::out_of_range&)